Dimensioning, Scaling, Sectioning

8/11/2019 Dimensioning, Scaling, Sectioning

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DIMENSIONING

Dimensions are used to supply detailed manufacturing, fabrication or construction information concerning the

size or location of the components of an object.

In applying dimensions to your drawing, you must familiarize yourself with these terminologies:

Dimension lines. Lines located between extension lines ending with an arrow and to include a numerical value.

They should be spaced 10 mm to 13 mm.

Extension lines. Extend away from a view to indicate a size or location constraint origin. When extension lines

cross object or extension lines, no gap in either line should be made.

Leader lines. Line drawn at an angle (never horizontal or vertical) extending from a note to a feature to which

the note applies.

Figure 1. Dimensioning terminologies.

Here are the simple things that you should remember when youre dimensioning:

1.

Always place shorter dimensions nearest to the object lines. Dimension lines should never cross but extension

lines may.

2.

Extension lines should never be shortened.

3.

Dimension lines are dimension lines and extension lines are extension lines. They shouldnt coincide nor be

substituted with other lines such as center line or object line.

Figure 2. Improper placement of dimension lines and extension lines.

4.

For neatness, dimensions should be lined up in a chain fashion or be grouped together as much as possible.


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Figure 3. Dimensions should be grouped together as much as possible.

5.

Do not repeat a dimension.

6.

Dimensions should be given in views where the shapes are presented in profile and/or contour.

Figure 4. The drawing on the left shows the correct way of dimensioning while the drawing on the right shows the wrong one.

7.

Dimensions should always be placed off or outside of a view when possible.


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Figure 5. Putting the dimensions inside the view is not allowed.

8.

Hole features must be located and given size in the view where they appear as a circle not as a rectangle or

hidden lines. Never dimension to hidden lines.

Figure 6. Size and location of holes should be shown when they appear as circles.

9.

Avoid a complete chain of dimensions. Either omit one or use reference notation. Reference notation indicates

that a dimension is used for information purposes only. Indicate a reference dimension by placing a parenthesis

around the dimension.


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Figure 7. Avoid complete chain of dimensions. Instead, use reference dimensions.

10.

Cylinder location and size constraint. Locate cylinders in the circular view but give its diameter and length in the

rectangular view.

Figure 8. Cylinder location and size constraint.

In dimensioning, we also use symbols to represent specific information which could be difficult to duplicate in

note form. They aid in clarity and saves time.


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Figure 9. Symbols used in dimensioning.

SCALING

We usually draw objects exactly the same size as the real one. But when dealing with very huge objects like

buildings which are designed by architects and civil engineers, we reduce the objects so that it will fit our paper.

Electronics engineers enlarge the drawings of integrated circuits so that they can look at its parts. These can be done

through scaling.

Scaling is a linear transformation that enlarges or shrinks objects by a scale factor that is the same in all

directions. Linear transformation means we will multiply the dimensions by the scale factor to obtain the scaled

dimensions.

In dimensioning, we will show the real dimensions not the scaled dimensions to prevent confusion.

The scale is shown as shown as the length in the drawing, then a colon (:), then the matching length on the

real thing. For example, we have a scale of 1:1000, then we could draw the Leaning Tower of Pisa (56 m high), 5.6 cm

high on our paper.

Figure 10. Leaning Tower of Pisa shown at 1:1000 scale.

The scale should normally be noted in the title block of drawing. When more than one scale is used they should

be shown close to the views to which they refer, and the title block should read as scales as shown. If a drawing uses

predominantly one scale it should be noted in the title block together with the wording or as shown.


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SECTIONING

If the drawing cannot fully show the interior detail of object, a sectional view is drawn to reveal the interior

detail. It is done by slicing your object with a cutting plane, removing the other part, and drawing the cut surface and

visible lines of the remaining part.

Figure 11. Sectioning is done by cutting the object with an imaginary plane.

Cutting plane is denoted by a chain line, thick at ends. Arrowheads indicate the direction in which the cutaway

object is viewed. Whenever a cutting plane coincides with a center line, the cutting plane takes precedence.

Figure 12. Cutting plane line.

All visible edges and contours behind the cutting plane should be drawn. Hidden lines should be omitted in

sectional views. Sometimes hidden lines, which are necessary for clearness, are drawn.


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Figure 13. Section view with hidden line for clarity.

Cut surface are filled with hatching lines. Hatching lines are thin lines and are drawn 45 degrees from the

horizon, left or right. For adjacent parts, hatching on one part should be at right angles to the hatching on the other part

For more than two adjacent parts, vary the angle and/or the spacing of hatching lines. Dimensions may be inserted in

hatching area by interrupting the hatching lines.

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Dimensioning, Scaling, Sectioning